Effects of photophysical properties of 1,4-cyclohexadiene derivatives on their [2 + 2] photocycloaddition reactivities: Experimental and theoretical studies

Article abstract of DOI:10.1016/j.jphotochem.2021.113336

To study the relationship between the [2 + 2] photocycloaddition reactivities of 1,4-cyclohexadiene derivatives (1,4?CHDs) and their structures, photophysical properties of a series of 1,4?CHDs were studied experimentally and by performing theoretical calculations. Specifically, UV–vis absorption spectra of these compounds in diluted solutions were acquired and the theoretical calculations were performed at the density functional theory (DFT) level. Their UV–vis absorption maxima were found to be related to the substituents on the 1,4-cyclohexadiene ring. To describe the [2 + 2] photocycloaddition reactivities of the 1,4?CHDs, time-dependent density functional theory (TDDFT) was used to optimize their ground- and excited-state structures, and their electronic excitation energies were calculated at the M062X/def-TZVP level. Frontier molecular orbitals and electron-hole distribution analyses were used to illustrate the electron transition modes of the 1,4?CHDs. The differences between the ground- and excited-state structures of the different 1,4?CHDs were characterized by carrying out a root-mean-square-deviation (RMSD) analysis. The results showed that the photophysical properties of 1,4?CHDs are meaningful for explaining their [2 + 2] photocycloaddition reactivities.

Full text of DOI:10.1016/j.jphotochem.2021.113336

Journal of Photochemistry & Photobiology, A: Chemistry 416 (2021) 113336
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Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Effects of photophysical properties of 1,4-cyclohexadiene derivatives on
their [2 + 2] photocycloaddition reactivities: Experimental and
theoretical studies
Xiaokun Zhang, Jingrui Cui, Hong Yan*
Beijing Key Laboratory of Environmental and Viral Oncology, Faculty of Environment and Life, Beijing University of Technology, Beijing, 100124, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
To study the relationship between the [2 + 2] photocycloaddition reactivities of 1,4-cyclohexadiene derivatives
1
,4-cyclohexadiene derivatives
(
1,4ꢀ CHDs) and their structures, photophysical properties of a series of 1,4ꢀ CHDs were studied experimentally
[
2+2] photocycloaddition reactivity
and by performing theoretical calculations. Specifically, UV–vis absorption spectra of these compounds in diluted
solutions were acquired and the theoretical calculations were performed at the density functional theory (DFT)
level. Their UV–vis absorption maxima were found to be related to the substituents on the 1,4-cyclohexadiene
ring. To describe the [2 + 2] photocycloaddition reactivities of the 1,4ꢀ CHDs, time-dependent density func-
tional theory (TDDFT) was used to optimize their ground- and excited-state structures, and their electronic
excitation energies were calculated at the M062X/def-TZVP level. Frontier molecular orbitals and electron-hole
distribution analyses were used to illustrate the electron transition modes of the 1,4ꢀ CHDs. The differences
between the ground- and excited-state structures of the different 1,4ꢀ CHDs were characterized by carrying out a
root-mean-square-deviation (RMSD) analysis. The results showed that the photophysical properties of 1,4ꢀ CHDs
are meaningful for explaining their [2 + 2] photocycloaddition reactivities.
Photophysical properties
Theoretical calculations
1
. Introduction
,4-Cyclohexadiene derivatives (1,4ꢀ CHDs) constitute one of the
levels, oscillator strength (f) levels, and geometrical and electronic
structures are closely related to the properties of their photochemical
1
reactions, and may be used to understand the process by which they
undergo [2 + 2] photocycloadditions [25].
most common organic structural units and are widely present in many
natural products and pharmaceutical molecules with physiological ac-
tivities [1–5]. They are used as raw materials or intermediates for syn-
thesizing important pharmaceutical and chemical materials [6–12].
Carrying out [2 + 2] photocycloadditions of 1,4ꢀ CHDs has been the
main approach used to synthesize tetraasteranes, which constitute an
important class of cage compounds with high levels of tension energy
and lipophilicity [13]. Their aza and oxa derivatives, namely 3,9-diaza-
tetraasteranes and 3,9-dioxatetraasteranes, display anti-HIV activity and
antitumor activity [14–18]. The tetraasteranes were first obtained via [2
To the best of our knowledge, there has been a lack of systematic
studies on the relationship between photophysical properties of
1,4ꢀ CHDs and their reactivities for [2 + 2] photocycloaddition.
1,4ꢀ CHDs were selected as the subjects of the current research, and
their [2 + 2] photocycloadditions were compared with that of each
other to clarify the different reactivities cause by the photophysical
properties. Specifically, five 1,4ꢀ CHDs were herein selected as repre-
sentative compounds for scrutinizing the effects of substituents of the
1,4-cyclohexadiene ring on the photophysical properties of the com-
pounds and on their [2 + 2] photocycloaddition reactivities (Scheme 1).
The ultraviolet-visible (UV–vis) absorption spectra of these compounds
in solution were acquired and are discussed in detail. To shed further
light on the nature of their photophysical behaviors, time-dependent
density functional theory (TDDFT) was used to optimize the ground-
and excited-state structures of the 1,4ꢀ CHDs. Frontier molecular or-
bitals and electron-hole distribution analyses were used to illustrate the
+
2] photocycloadditions of 1,4ꢀ CHDs by Cookson and Hans-Martin
[
19,20]. They were at that time considered as novel tetraasterane de-
rivatives, and found to form head-to-tail (HT) cage-dimers of 3,6-dihy-
drophthalic anhydride with relatively low yields (of about 10 %) and
low levels of regioselectivity. In view of our studies on the [2 + 2]
photocycloaddition reactivities of 1,4-dihydropyridines and 4H-pyrans
[
21–24], their photophysical properties such as excitation energy (E)
*
Corresponding author.
E-mail address: hongyan@bjut.edu.cn (H. Yan).
https://doi.org/10.1016/j.jphotochem.2021.113336
Received 11 March 2021; Received in revised form 28 April 2021; Accepted 2 May 2021
Available online 4 May 2021
1
010-6030/© 2021 Elsevier B.V. All rights reserved.

X. Zhang et al.
Journal of Photochemistry & Photobiology, A: Chemistry 416 (2021) 113336
electron transition modes of the 1,4ꢀ CHDs. Their electronic excitation
energies were calculated at the M062X/def-TZVP level; meanwhile, the
differences between the ground- and excited-state structures of the
different 1,4ꢀ CHDs and the intrinsic characteristic of excitations were
characterized by carrying out wavefunction analyses. The results have
provided comprehensive insight into the photophysical properties and
O
COOH
+
O
THF
90ºC
COOH
O
[
2 + 2] photocycloaddition reactivities of the 1,4ꢀ CHDs.
1
2
. Materials and methods
O
O
2
.1. General procedures
O
N H HCl
2
4
NH
NH
The solvents and reagents were used as received without further
O
purification. Proton NMR spectroscopy data were recorded using a
Bruker AV400 spectrometer operating at 400 MHz with TMS used as an
internal standard at ambient temperature (25 ℃). The UV–vis absorp-
tion spectra for the compounds, in respective 1,4-dioxane solutions,
were acquired using a SHIMADZU UV-2600 spectrophotometer. All
AcOH
reflux
O
2
Scheme 2. Synthesized protocol for preparing 1,4ꢀ CHDs 1-2.
experiments were performed at 25 ℃ and the concentrations of
,4ꢀ CHDs 1-4 and 1,4ꢀ CHDs 5 are 4 × 10ꢀ 4 M and 4 × 10 M,
-5
1
confirmed attainment of local geometric minima. The solvent effects
were investigated using the polarizable continuum model (PCM) [30,
respectively.
3
1] to simulate the 1,4-dioxane environment. TDDFT [32,33], which is
2
.2. Synthesis of 1,4-cyclohexadiene derivatives
the most popular method for these purposes due to its desirable balance
between computational efficiency and accuracy, was employed here to
calculate the electronic absorption spectra on the basis of the optimized
geometries of the 1,4ꢀ CHDs. All wavefunction analyses were conducted
using the Multiwfn 3.8 program [34] and the isosurface maps of
electron-hole distribution were rendered with VMD 1.9.3 software [35].
All calculations were finished using the Gaussian 09 packages [36].
1
,4ꢀ CHDs 1 and 2 were prepared by the synthetic protocol pre-
sented in Scheme 2 and 1,4ꢀ CHDs 3–5 were commercial compounds
1
[
26,27]. All of them were verified by H NMR.
,7-dihydroisobenzofuran-1,3-dione (1): 1,3-Butadiene (180
mmol) and acetylenedicarboxylic acid (100 mmol) were added in THF
100 mL). The reaction mixture was heated at 90 ℃ for 12 h in a glass
4
(
pressure bottle. Then the mixture was cooled down to room temperature
and removed the solvent by distillation under reduced pressure and then
recrystallized from ethyl acetate. Yield: 86 %; White solid; m.p. 148 ℃;
3
. Results and discussion
Five 1,4ꢀ CHDs, with different substituents on the 1,4-cyclohexa-
1
H NMR: (400 MHz, Chloroform-d) δ 5.90 (d, 2 H), 3.14 (d, 4 H).
diene ring, were selected as representative compounds for scrutinizing
the effect of photophysical properties on [2 + 2] photocycloaddition
reactivity. The 1,4ꢀ CHDs 1, having a dicarboxylic anhydride substitu-
ent on the 1,4-cyclohexadiene ring, has been shown to be able to un-
dergo an intermolecular [2 + 2] photocycloaddition to form the novel
tetraasterane derivative tetraasterantetracarbonacidanhydride [19].
The 1,4ꢀ CHDs 2 was designed by us as an aza derivative of the 1,
2
,3,5,8-tetrahydrophthalazine-1,4-dione
(2):
Crystals
of
1
,4ꢀ CHDs 1 (37.5 mmol) were crushed in a mortar, and then added to a
refluxing solution of NH NH
2
2
⋅HCl (84 mmol) in AcOH (200 mL). The
resulting mixture was refluxed overnight to yield a colorless, clear so-
lution. The reaction mixture was cooled and poured into 700 mL of
water. The resulting precipitate was filtered and dried under reduced
pressure to yield 1,4ꢀ CHDs 2 as a white solid. Yield: 98 %; White solid;
4
ꢀ CHDs 1, with the idea of enhancing the conjugation between the
1
m.p. 156 ℃; H NMR (400 MHz, DMSO-d
6
) δ 11.59 (s, 2 H), 5.81 (s, 2 H),
–
–
carbonyl groups and the C C double bonds in the 1,4-cyclohexadiene
2
.99 (s, 4 H).
ring [37,38]. 1,4ꢀ CHDs 3 and 4 were each made by carrying out a
ring opening of the substituent of the 1,4ꢀ CHDs 1. To date, there have
been no reports on the [2 + 2] photocycloadditions of 1,4ꢀ CHDs 2 and
1
Dimethyl cyclohexa-1,4-diene-1,2-dicarboxylate (3): H NMR (400
MHz, Chloroform-d) δ 5.67 (d, 2 H), 3.75 (s, 6 H), 2.97 (d, 4 H).
Dimethyl bicyclo [2.2.1] hepta-2,5-diene-2,3-dicarboxylate (4):
3
4
4
. When attempts at carrying out [2 + 2] photocycloadditions of 1,
ꢀ CHDs 2 and 3 were made based on the reaction conditions used for 1,
ꢀ CHDs 1 and 5 (see below), no cycloaddition products except for their
1
H NMR (400 MHz, Chloroform-d) δ 6.93 (t, 2 H), 3.95 (p, 2 H), 3.79 (s,
6
H), 2.29 (dt, 1 H), 2.11 (dt, 1 H).
2
.5-Dimethyl-1,4-benzoquinone (5): 1H NMR (400 MHz, Chloro-
form-d) δ 6.67 (q, 1 H), 1.97 (d, 3 H).
aromatization products were obtained. The 1,4ꢀ CHDs 4 can undergo an
intramolecular [2 + 2] cycloaddition [39]. And the 1,4ꢀ CHDs 5 has
been shown to easily undergo intermolecular [2
+
2] photo-
2
.3. Computational details
cycloaddition
to
5.10
form
dodecane [20]. These experimental results
Scheme 3) revealed very different [2 + 2] photocycloaddition re-
activities for the 1,4ꢀ CHDs with different substituents.
the
1,4,5,8-tetramethylpentacyclo
6.4.0.02 .04.11.0
.7
]
[
Molecular geometry optimizations and vibrational analyses of the
(
ground and excited states of the compounds each in 1,4-dioxane were
carried out using the M062X functional [28] associated with the
def-TZVP basis set [29]. The absence of an imaginary frequency
Scheme 1. Structures of the examined 1,4ꢀ CHDs.
2

X. Zhang et al.
Journal of Photochemistry & Photobiology, A: Chemistry 416 (2021) 113336
are displayed in Fig. 1 with the relevant data in Table 1, and with λabs
denoting wavelength of maximum absorption (nm), E denoting excita-
tion energy (eV), calculated using the equation E = 1240/λabs, and
ε
denoting molar extinction coefficient.
As shown in Fig. 1, the absorption maxima of 1,4ꢀ CHDs 1–5 were
located in the range 245ꢀ 301 nm with molar extinction coefficient (
ε
)
3
ꢀ 1
ꢀ 1
values of about 10 M cm . These results indicated the lowest-energy
* tran-
excited states of 1,4ꢀ CHDs 1–5 to be influenced mainly by
π-π
sitions. The five 1,4ꢀ CHDs tested showed five different λabs values as
shown in Table 1, indicating the effects of the substituents on λabs. The
λ
abs values of the 1,4ꢀ CHDs 1 (289 nm) and 2 (301 nm) were observed
to be similar and the differences between the λabs values of 1,4ꢀ CHDs
1
–2 and those of 3-4 were large, namely above 30 nm, indicating a
bathochromic effect of cyclizing the substituent. These results suggested
the occurrence of an enhancement of the conjugation between the
–
carbonyl groups of the substituents and the C C double bonds in the
–
main 1,4-cyclohexadiene ring as a result of the cyclization of the sub-
stituents. The carbonyl groups in 1,4ꢀ CHDs 1–2 were also indicated
from our current results to be engaged. The aza analogue of the dicar-
boxylic anhydride in the 1,4-cyclohexadiene ring would reduce the
excitation energies of 1,4ꢀ CHDs, and the λabs value of 1,4ꢀ CHDs 2 was
observed to be higher than that of 1,4ꢀ CHDs 1, which may have been
due to the particularly good conjugation of the lone-pair electrons of the
nitrogen atoms with the carbonyls. The λabs value of 1,4ꢀ CHDs 5 (262
nm) was observed to be higher than those of 1,4ꢀ CHDs 3–4 and lower
than those of 1,4ꢀ CHDs 1–2. The carbonyls at positions 3 and 6 of the
1
2
1
,4-cyclohexadiene ring may be the main reason for the red shift (about
0 nm) of the absorbance of 1,4ꢀ CHDs 5. Comparison of the value of
,4ꢀ CHDs 5 (13000 M cm ) and those of 1–4 indicated that the
ε
ꢀ 1
ꢀ 1
inclusion of carbonyls attached to positions 3 and 6 of the 1,4-cyclohex-
adiene ring would bring about a higher . This observation was found to
be in good agreement with experimental results that showed that the [2
2] photocycloaddition of the 1,4ꢀ CHDs 5 could occur in sunlight with
high yield [20].
ε
+
Scheme 3. [2 + 2] photocycloadditions of 1,4ꢀ CHDs.
.1. UV–vis absorption spectra of 1,4ꢀ CHDs 1–5
3
.2. Theoretical calculations
3
Time-dependent density functional theory (TDDFT) was used to
optimize the ground- and excited-state structures of 1,4ꢀ CHDs 1–5.
Their electronic excitation energies at the M062X/def-TZVP level were
calculated so as to gain further insight into their optical and electronic
properties in their ground and excited states. First the effects of sub-
stituents of the 1,4ꢀ CHDs on their geometrical and electronic structures
in the ground states were investigated, based on the optimized
geometrical structures in 1,4-dioxane. Then the frontier molecular or-
bitals and electron-hole distribution analyses were carried out to shed
light on the intrinsic characteristics of their electron excitation proper-
ties. Finally, the structures of the excited state and ground state minima
of the different 1,4ꢀ CHDs were compared in order to predict the [2 + 2]
photocycloaddition reactivities of 1,4ꢀ CHDs.
For wavelengths of 225ꢀ 400 nm, UV–vis absorption spectra of
,4ꢀ CHDs 1-5 each in 1,4-dioxane were acquired. All experiments were
1
performed at 25 ℃ and the concentrations of 1,4ꢀ CHDs 1-4 and
,4ꢀ CHDs 5 are 4 × 10ꢀ M and 4 × 10 M, respectively. The spectra
4
-5
1
3
.2.1. Geometrical and electronic structures of the ground states
The optimized ground-state geometrical structures of 1,4ꢀ CHDs
each in 1,4-dioxane are presented in Fig. 2. Their bond length (BL)
values are presented in Table 2. Apart from the bond lengths, the Lap-
lacian bond order (LBO) values were also calculated, due to the excellent
Table 1
Summary of UV–vis absorption spectroscopy data of 1,4ꢀ CHDs 1-5.
ꢀ
1
ꢀ 1
1
,4-CHDs
λ
abs/nm
E/eV
ε/ M cm
1
2
3
4
5
289
301
250
245
262
4.29
4.12
4.96
5.06
4.73
1664
2720
1445
4516
13000
Fig. 1. UV–vis spectra of 1,4ꢀ CHDs 1-5 in 1,4-dioxane.
3

X. Zhang et al.
Journal of Photochemistry & Photobiology, A: Chemistry 416 (2021) 113336
Fig. 2. Optimized ground-state geometrical structures at the M062X/def-TZVP level.
Table 2
Values for bond length (BL, Å) and Laplacian bond order (LBO, Å) of 1,4ꢀ CHDs
in the ground states in 1,4-dioxane at the M062X/def-TZVP level.
Index
1
2
3
4
5
1,4-cyclohexadiene
BL1(C
BL2(C
BL3(C
BL4(C
BL5(C
BL6(C
LBO1
LBO2
LBO3
LBO4
LBO5
LBO6
1
-C
2
-C
3
-C
4
-C
5
-C
6
-C
2
)
3
)
4
)
5
)
6
)
1
)
1.340
1.490
1.513
1.337
1.513
1.490
1.801
1.057
1.015
1.844
1.015
1.057
1.352
1.504
1.504
1.334
1.504
1.504
1.721
1.023
1.053
1.860
1.053
1.023
1.344
1.511
1.502
1.333
1.502
1.511
1.746
0.991
1.053
1.860
1.053
0.992
1.335
1.534
1.538
1.328
1.537
1.538
1.712
0.848
0.868
1.850
0.873
0.829
1.347
1.479
1.499
1.347
1.479
1.499
1.798
1.193
1.109
1.798
1.193
1.109
1.327
1.502
1.502
1.327
1.502
1.502
1.855
1.049
1.049
1.855
1.049
1.049
quantitative relationship between LBO and chemical bond strength
40], and are presented in Table 2. Inspired by previous works [25,41],
relative deviation of bond length (RDBL) and Laplacian bond order
RDLBO) were defined and calculated to investigate the effects of the
[
(
substituents on geometrical and electronic structures. The results are
presented in Fig. 3 with 1,4-cyclohexadiene as a reference compound.
As shown in Fig. 3, the substituents were indicated to have an
obvious influence on the bond length (BL) and bond order (LBO) values
of 1,4ꢀ CHDs. To be specific, the lengths of the 1,4
bonds BL1 (C -C ) and BL4 (C -C
increased upon introduction of the substituents by about 1.2 % and 0.8
–
CHDs CC
–
double
1
2
4
5
) were calculated to have been
%
, respectively, with this increase attributed to the conjugation between
–
–
the carbonyl groups and the C C double bonds. As a result, LBO1 and
LBO4 were weakened by about 5.0 % and 2.0 %, respectively. In
particular, the RDBL of BL1 (1.9 %) of the 1,4ꢀ CHDs 2 was the largest for
the aza ring substituent and was attributed to the low ring tension. The
BL2, BL3, BL5 and BL6 values of the 1,4ꢀ CHDs 4 were clearly increased
relative to those of 1,4ꢀ CHDs (by about 2.0 % of RDBL) due to the in-
clusion of the carbon bridge while the BL1 and BL4 values were almost
unchanged (with a change of only about 0.3 % of RDBL). The results
showed the carbon bridge and ring substituents providing the greatest
contributions to the conjugation in the 1,4ꢀ CHDs. Those 1,4ꢀ CHDs
Fig. 3. Bond length (BL) and Laplacian bond order (LBO) values of the
–
double bonds with the lowest bond orders were found
having the CC
–
1
,4ꢀ CHDs in their ground states.
to have the lowest excitation energies and to most easily undergo
photochemical reactions. The results showed the carbon bridge and ring
substituents providing the greatest contributions to the reducing of
4

X. Zhang et al.
Journal of Photochemistry & Photobiology, A: Chemistry 416 (2021) 113336
excitation energies. The presence of the carbonyls in the 1,4ꢀ CHDs 5
Table 3. To evaluate the structural distinctions between the excited state
(ES) and ground state (GS) minima of 1,4ꢀ CHDs 1–5, here we compared
the relative deviation values for bond length (RDBL) and Laplacian bond
order (RDLBO) calculated using a method similar to the method used in
the above section, and the results are displayed in Fig. 6. Both the var-
iations of bond length and Laplacian bond order of 1,4ꢀ CHDs 2–4
showed similar trends. The BL1 and BL4 values were calculated to be
increased by 4.0 %; while in contrast, the BL3 and BL5 values were
shortened by 3.0 %. For 1,4ꢀ CHDs 1 and 5, BL1 and BL4 were calculated
to be just slightly changed with the relative deviation values within 5.0
was indicated to lead to moderate increases in BL1 and BL4 (by about
1
1 6
.6 % of RDBL), while BL3 and BL6 (C -C ) showed lower sensitivity to
the presence of the carbonyl (RDBL values of about 0.1 %). These ob-
servations were in good agreement with experimental results showing
that [2 + 2] photocycloaddition of the 1,4ꢀ CHDs 1 could occur in light
of longer wavelengths, and the carbon bridge reducing the distance
between the two C–
–
C double bonds of the 1,4ꢀ CHDs 4 was indicated to
allow 4 to easily undergo the intramolecular [2 + 2] photocycloaddition
19,39].
[
%
accompanied by fluctuation of bond strength of about 1.0 %, indic-
3
.2.2. Theoretical calculations for electron excitation
ative of their being insensitive to excitation. According to the relative
deviation values presented in Fig. 6, the RDLBO of LBO3 and LBO5 were
both increased by the same amount, namely 25.0 %, together with a
contraction of BL1 and BL4 by 25.0 % and 15.0 %, respectively. In
To shed light on the intrinsic characteristics of electron excitation
processes, molecular orbitals (MOs) were used to illustrate the electron
transition modes of these compounds, due to the overwhelming contri-
–
butions of the S
0
→S
1
transitions; depictions of the HOMOs and LUMOs
contrast, the strengths of the C C double bonds in the 1,4-cyclohexa-
–
of 1,4ꢀ CHDs 1-5 are shown in Fig. 4.
diene ring were indicated to be weakened significantly after excita-
tion, and thus resulting in enhancements of their chemical reactivities;
these double bonds have been shown to spatially coincide well with the
locations of the reactive sites of their photoreactions, especially the [2 +
2] photocycloaddition.
All of the frontier orbitals of these compounds were indicated to be
mainly of the -type and to be predominately located on the both sides of
π
the 1,4-cyclohexadiene ring, hence showing a considerable separation in
space. That indicated the lowest-energy transition of the 1,4ꢀ CHDs 1–3
to principally be a charge transfer (CT) excitation (Fig. 4). While the
distribution of the frontier orbitals was indicated to be affected by the
carbonyl groups at positions 3 and 6 of the 1,4ꢀ CHDs 5, the excitation
showed an apparent local excitation (LE) mode. Besides, apparent
changes of the LUMO distribution brought about by the carbon bridge of
the 1,4ꢀ CHDs 4 were noticed as well, and apparently led to delocal-
ization of LUMO and hence its spread to the other C–
Additionally, superpositions of the ES- and GS-minima structures and
their RMSD values (Å) for each of the 1,4ꢀ CHDs 1–5 are shown in Fig. 7.
In particular, the structures of the first triplet states were also included;
they have an important influence on the photochemical reactions of
1,4ꢀ CHDs. For the 1,4ꢀ CHDs 1 as well as for the 1,4ꢀ CHDs 2, these
RMSDs were quite small and the structures were quite planar regardless
of the state, attributed to restraints imposed by the dicarboxylic anhy-
dride and aza substituents; thus each of them was expected to be able to
attack another molecule without steric hindrance and undergo inter-
molecular [2 + 2] photocycloaddition. Nevertheless, for the 1,4ꢀ CHDs
2, no [2 + 2] photocycloaddition product has yet been produced, the
reasons for which need still to be determined. A larger RMSD value was
calculated for the 1,4ꢀ CHDs 3 than for the other 1,4ꢀ CHDs, illustrating
an obvious distinctive feature in the structure of 3 and in accord with its
considerable steric hindrance, the main reason for the lack of any
observed [2 + 2] photocycloaddition of 3. In addition, the carbon bridge
–
C double bond in
our calculations. Comparing the distributions of the frontier orbitals of
1
,4ꢀ CHDs 1–2 and 1,4ꢀ CHDs 3–5 showed some characteristics of their
HOMOs and LUMOs having spread to the introduced dicarboxylic an-
hydride substitutes and aza derivative. In addition, to reveal the char-
acteristics of the electron excitations more intuitively, the calculations of
the electron-hole distributions of 1,4ꢀ CHDs 1–5 after excitation were
carried out (Fig. 5).
Inspection of these plots showed the lowest-energy transitions of
1
,4ꢀ CHDs 1–3 mainly being charge transfer excitation (CT), and those
of 1,4ꢀ CHDs 4–5 mainly being local excitation (LE). Apparently, apart
of the 1,4ꢀ CHDs 4 also apparently prevented any considerable change
* transition induced by electron excitation of C–
C double
in its structure, and the distance between the two C
–
–
C bonds was
from the
π
-π
–
bonds, the n-
atom was also incorporated in the S
,4ꢀ CHDs 1–5, an additional * excitation was found from the C–H
π
* transition from the lone-pair electrons of the oxygen
shortened. This analysis confirmed the experimental results showing the
occurrence of only an intramolecular [2 + 2] photocycloaddition of 4.
For the 1,4ꢀ CHDs 5, the presence of the conjugated carbonyls effec-
tively maintained the planarity of the 1,4-cyclohexadiene ring structure,
consistent with 5 having been observed to readily undergo an inter-
molecular [2 + 2] photocycloaddition.
0
→S transition. Furthermore, for
1
1
σ-π
single bond, and this finding explained the relatively high excitation
energies of 1–5. These observations were in good agreement with
experimental results showing the excitation energies levels of 1,4ꢀ CHDs
to be high (UV range) and the results showing that the
ε
value of the
1
,4ꢀ CHDs 5 was higher than those of 1,4ꢀ CHDs 1–4 because of the
4. Conclusions
local excitation mode.
In summary, an experimental and theoretical study of five 1,4-cyclo-
hexadiene derivatives (1,4ꢀ CHDs) was carried out for the purpose of
investigating their photophysical properties with regards to their [2 + 2]
photocycloaddition reactivities. The 1,4-cyclohexadiene ring of
3
.2.3. Geometrical and electronic structures of the excited states
The bond length (BL) and Laplacian bond order (LBO) values for the
,4-cyclohexadiene rings in their relaxed excited states are displayed in
1
Fig. 4. Plots of HOMOs and LUMOs (isosurface = 0.05 a.u.) of 1,4ꢀ CHDs 1-5.
5

X. Zhang et al.
Journal of Photochemistry & Photobiology, A: Chemistry 416 (2021) 113336
Fig. 5. Electron-hole distributions (isosurface = 0.002 a.u.) of 1,4ꢀ CHDs 1-5 after excitation. The green regions represent increases in electron density as a result of
excitation and the blue regions represent depletions of electron density.
Table 3
Bond length (BL, Å) and Laplacian bond order (LBO, Å) values for the 1,4-cyclo-
hexadiene ring in their relaxed excited states in 1,4-dioxane as calculated at the
M062X/def-TZVP level.
Index
1
2
3
4
5
BL1
1.366
1.491
1.507
1.328
1.507
1.491
1.544
1.009
1.019
1.848
1.019
1.009
1.410
1.476
1.451
1.390
1.451
1.476
1.287
1.086
1.261
1.606
1.261
1.087
1.411
1.504
1.454
1.390
1.454
1.504
1.297
1.008
1.261
1.620
1.261
1.008
1.425
1.554
1.486
1.407
1.485
1.558
1.249
0.734
1.082
1.513
1.093
0.722
1.340
1.462
1.475
1.340
1.462
1.475
1.742
1.091
1.043
1.742
1.091
1.043
BL2
BL3
BL4
BL5
BL6
LBO1
LBO2
LBO3
LBO4
LBO5
LBO6
1
1
1
1
,4ꢀ CHDs
1
included a dicarboxylic anhydride substituent. The
,4ꢀ CHDs 2 was designed as an aza derivative of 1,4ꢀ CHDs 1,
,4ꢀ CHDs 3 and 4 as ring-opened derivatives of 1,4ꢀ CHDs 1, and
,4ꢀ CHDs 5 as a p-benzoquinone derivative.
Inspection of the UV–vis absorption spectra of the 1,4ꢀ CHDs showed
significant effects of the substituents on λabs. The λabs values of the
1
,4ꢀ CHDs 1 (289 nm) and 2 (301 nm) were observed to be similar, and
those of 1,4ꢀ CHDs 3 (250 nm) and 4 (245 nm) were also found to be
close to each other. These results suggested the occurrence of an
enhancement of the conjugation between the carbonyl groups of the
substituents and the C–
–
C double bonds in the main 1,4-cyclohexadiene
ring as a result of the cyclizations of the substituents. The λabs value of
1
3
3
,4ꢀ CHDs 5 (262 nm) was found to be longer than those of 1,4ꢀ CHDs
–4 and shorter than those of 1,4ꢀ CHDs 1–2. The carbonyls at positions
and 6 on the 1,4-cyclohexadiene ring may be the main reason for the
red shift (of about 20 nm) of 1,4ꢀ CHDs 5.
Theoretical calculations of geometrical and electronic structures in
the ground states revealed the carbon bridge and ring substituents
providing the greatest contributions to the reducing of excitation energy
of the 1,4ꢀ CHDs. A frontier molecular orbital and electron-hole distri-
bution analysis revealed the lowest-energy transition of 1,4ꢀ CHDs 1–4
to be principally a charge transfer excitation and the distribution of the
frontier orbitals to be affected by the presence of carbonyls at positions 3
and 6 of 1,4ꢀ CHDs 5, indicated to have an apparent local excitation
Fig. 6. Bond length (BL) and Laplacian bond order (LBO) values of the relaxed
excited states (ES-minima) and ground states (GS-minima) of the five
1
,4ꢀ CHDs each in 1,4-dioxane. Calculations were performed at the M062X/def-
TZVP level.
6

X. Zhang et al.
Journal of Photochemistry & Photobiology, A: Chemistry 416 (2021) 113336
Fig. 7. Superpositions of the ES- and GS-minima structures and their RMSD values (Å) for each of the 1,4ꢀ CHDs 1-5.
mode. Geometrical and electronic structures of the excited states were
analyzed by calculating root-mean-square-deviations (RMSDs). The
substituents were shown from these calculations to have a significant
influence on the variations of the structures of the excited states. Re-
straints provided by the dicarboxylic anhydride and aza derivative
substituents were concluded to maintain the planarities of the
Acknowledgments
This work was financially supported by the 2019 Beijing Natural
Science Foundation (No. 2192004).
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